1. Field of the Invention
This invention relates generally to the mooring of floating structures, and more particularly, but not by way of limitation, to the mooring of a floating structure utilized as an offshore oil and gas drilling and production platform.
2. Description of the Prior Art
As offshore exploration for oil and gas from subsea deposits has expanded into deeper and deeper waters, conventional rigid towers setting upon the ocean floor and extending upward to the surface have become more and more impractical.
One particular solution to this problem is the elimination of the rigid tower and the substitution therefor of a floating platform moored to the ocean floor by a plurality of vertical members which are placed under high tension loads due to excess buoyancy of the floating platform. Examples of such structures, which are generally referred to as tension leg platforms; are shown in U.S. Pat. No. 3,648,638 to Blenkarn and U.S. Pat. No. 3,919,957 to Ray, et al.
One particular problem which must be overcome with any design of tension leg platform is the manner in which the floating platform is attached to the subsea anchor, i.e. the manner of mooring. Particularly, when locating the platform in deep waters where severe environmental conditions are often present, such as for example in the North Sea, it is desirable that the actual mooring of the platform be accomplished in a relatively short time, e.g. a matter of hours, and that it be accomplished without the need for the use of divers.
One manner of achieving these ends is suggested by U.S. Pat. No. 3,919,957 to Ray, et al. and U.S. Pat. No. 3,982,492 to Steddum. Both of those references, which disclose substantially the same structure, use dead weight anchors which are lowered from the floating platform to the ocean floor by the vertical tethering elements.
Another system is proposed in U.S. Pat. No. 3,976,021 to Blenkarn et al. and in U.S. Pat. No. 4,062,313 to Stram, wherein a gravity base is lowered on temporary mooring cables from the floating platform and is then attached to the ocean floor by a plurality of piles. Then the permanent tethering elements are lowered from the floating platform and attached to the anchors. The path of the permanent tethering elements as they are lowered into engagement with the anchors is defined by a plurality of guide members, having guide openings 82 such as shown in FIG. 12 of Blenkarn et al., and vertical guide passages 41 located upon the anchor as shown in FIG. 9 of Blenkarn et al. Both of the openings 82 and 41 of Blenkarn et al. include upwardly opening funnel shaped portions. The platform of Blenkarn et al. is moved into place with tug boats. After the risers are connected, the temporary mooring cables are released.
Yet another manner of solving the problem of connecting the permanent tethering elements between the anchor and the floating platform is to manufacture the permanent tethering elements integrally with the platform and then attach the tethering elements and the platform to the anchor in one step as is disclosed in U.S. Pat. No. 3,611,734 to Mott.
In U.S. Pat. No. 3,955,521 to Mott, individual tethering elements are lowered into engagement with pre-set anchor piles.
Other references relating generally to tension leg platforms and/or the lowering of anchors or other objects from floating structures, but not believed to be any more relevant than the references discussed in more detail above, include:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 4,126,008 Dixon 4,181,453 Vache 4,169,424 Newby, et al. 4,129,009 Jansz 4,127,005 Osborne 3,996,755 Kalinowski 3,986,471 Haselton 3,943,725 Pennock 3,654,886 Silverman 3,572,044 Pogonowski 4,109,478 Gracia 3,672,177 Manning 4,039,025 Burkhardt, et al. ______________________________________